Add patroni_use_kubernetes parameter.

Add a dedicated parameter to enable support for Kubernetes in Patroni
instead of relying on a legacy hack of setting etcd_host to an empty
string. The latter still works, however, it will show a warning, nudging
users to change to the new parameter.

For the sake of not breaking existing non-Kubernetes setups,
patroni_use_kubernetes has not effect if etcd_host is set to a non-empty
value.